Electrical insulating oil is used in high-tension electric devices for power transmission such as, for example, oil-immersed transformers, high-tension oil-immersed cables, oil circuit breakers, and oil condensers. While in service in such devices, the electrical insulating oil is required to retain its electrical properties intact and refrain from corroding metallic materials of the devices over long spans of time.
The recent introduction of a technique for the transmission of electric power of extra-high to super-high voltage ranging from 500,000 volts to 1,000,000 volts for the purpose of ensuring economic high-capacity power transmission has given great vigor to the demand for an electrical insulating oil of greater stability to resist oxidation.
As criteria for the evaluation of the oil's stability performance to resist oxidation, JIS C-2320 (Japanese Industrial Standard, concerning "electrical insulating oil") specifies that the total acid number of the oil should not exceed 0.6 mg KOH/g, and the sludge content of the oil should not exceed 0.4 wt% (as measured by the method for testing of oxidation stability specified by JIS C-2101). Non-additive type electrical insulating oils commercially available in Japan are such that their total acid numbers fall in the range of about 0.2 to 0.5 mg KOH/g. Accordingly, these oils still have room for improvement with respect to their ability to resist oxidation.
Criteria available for the evaluation of the quality of an electrical insulating oil are offered by ASTM D-3487, ASTM D-1040, ASTM D-1818, and ASTM D-1819. ASTM D-2440 which covers a testing method for the oxidation stability (similar to the specification of JIS C-2101) specifies that the upper limit of the acid number should not exceed 0.3 mg KOH/g (providing that ASTM D-387 allows addition of an antioxidant in Type II). By reason of electrical properties, electrical insulating oils of the non-additive type are exlusively meeting the demand in the market.
In electrical insulating oils of mineral oil type heretofore known to possess good oxidation stability performance, total nitrogen contents in the oils are limited to within the order of several ppm's (as indicated in Japanese Patent Application (OPI) No. 40799/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")).
U.S. Pat. No. 3,759,817 discloses an invention which requires that the total nitrogen content not exceed 20 ppm and the basic nitrogen limited to within not more than 5 ppm. The idea of improving the oxidation stability of the electrical insulating oil by positively causing the oil to contain therein a non-basic nitrogen content has never been reported or even suggested in literature.
Nitrogen compounds contained in the lubricant fraction separated from a crude oil have been considered as undesirable substance for oxidation stability. Efforts have been made to thoroughly remove the nitrogen compounds as to improve the oxidation stability by treating the fraction such as hydrogenation refining, solvent extraction refining, and solid adsorption refining, occasionally as combined with refining by sulfuric acid treatment.
In the case of crude oil of Middle East origin, for example, the light lubricant fraction separated by distillation from the crude oil has usually a total nitrogen content in the range of about 350 to 450 ppm.
When the lubricant fraction is refined by extraction from such a solvent as furfural, phenol, or N-methyl-2-pyrrolidone, it is divested of sulfur compounds and nitrogen compounds as well as polycyclic aromatic hydrocarbons. When this refining is carried out in combination with hydrogenation refining and solid adsorption refining, there is obtained highly refined mineral oil of notably improved oxidation stability. For all the efforts, the quality of this mineral oil is such that the acid number determined by the test for oxidation stability (JIS C-2101) is barely on the order of 0.2 to 0.3 mg KOH/g. The refining operation described above is not capable of affording mineral oil of better quality having an acid number of the order of 0.05 to 0.1 mg KOH/g. Worse still, this refining operation suffers from higher refining cost and lower yield.
An electrical insulating oil which consists of either an alkylbenzene alone or a mixture of a mineral oil with not less than about 20% by volume of an alkylbenzene as a base oil is highly effective in preventing the drift charging as well as in lowering the pour point but is deficient with respect to its oxidation stability as compared with an electrical insulating oil of the type using mineral oil alone.